Use `fileNameFromUri` utility method

[migueltylee]

The methods .split('/').last are commonly used on strings in several places of the DevTools codebase. It is preferred to use the utility method fileNameFromUri instead. TODO: https://github.com/flutter/devtools/blob/7dba4d6809ad6e0eb8105c576a92517e4a2378cb/packages/devtools_app/lib/src/primitives/utils.dart#L1461